/Engine/Source/Runtime/UMG/Public/Blueprint/WidgetTree.h
Iterates through all child widgets including widgets contained in named slots, other than investigating named slots, this code does not dive into foreign WidgetTrees, as would exist inside another user widget.
static void ForWidgetAndChildren ( [UWidget](API\Runtime\UMG\Components\UWidget) * Widget, [TFunctionRef](API\Runtime\Core\GenericPlatform\TFunctionRef)< void([UWidget](API\Runtime\UMG\Components\UWidget) *)> Predicate )